This invention relates to control units for industrial power applications in which high voltage, high amperage single or three-phase power is supplied to a using system and, more particularly, to a method of controlling application of power by the controller.
Power controllers are used in industrial applications for example, to supply power to machinery, manufacturing equipment, support systems such as heating and air conditioning, etc. For a particular installation, the controller may handle three-phase voltages of between 208Vac to 600Vac, with currents ranging from 50A to 2500A. The power controller provides the interface between the power supplied by an electrical utility, user owned power generating facility, or other supplier, and the electrical distribution network being served. The controller functions to supply power to using equipment under normal operating conditions, as well as interrupt supply of power in the event of overloads or other extraordinary circumstances, to prevent damage to the network and the equipment connected to it. Power controllers typically involve large, expensive installations which include numerous switches, sensors, and indicators by which power can be automatically distributed through the network. The controller, for example, includes a silicon controlled rectifier (SCR) type of power controller, as is well-known in the art. The controller is preceded by either a circuit breaker or an automatic disconnect switch. The power controller is wired to the circuit breaker or disconnect switch and this connection usually requires between two feet and five feet (0.61m-1.27m) of cabling. There is not a straight run between the circuit breaker or switch and the power controller; rather, to preserve space, the cables are bent. However, given the size and length of the cables, even this type of installation takes up considerable space. And, the cost of the controller includes the cost required to install the cabling in place.
Most large electrical resistance heaters utilize a three-phase circuit. Each circuit is individually fused. The SCR type power controller then regulates the amount of electricity supplied to the heater. SCR's, as is well-known in the art, are gated on to allow current flow in a particular direction. Once a SCR is gated into conduction, it continues conducting until current flow in the desired direction stops. For alternating current power distribution networks, each SCR must be gated into conduction for each half-cycle of the AC input wave form during which current flows to a particular phase. The output of the power controller is first connected to a power distribution unit, and then to a number of three-phase fuse blocks. For example, the output may be wired to six or eight of such blocks. Installation of the distribution unit and fuse blocks consumes between eight and fifteen square feet (2.9m.sup.2 - 5.5m.sup.2) of enclosure space. This is a substantial amount of volume. In addition to the power distribution unit and fuse blocks, the controller also includes a firing unit or firing package. This firing package has outputs connected to the respective gate inputs of each SCR. Control or gate inputs for the SCR's are supplied as inputs to the circuitry within the firing package which, in turn, produces the gating signals supplied to the SCR's to gate them into conduction at the proper times. Further, if there is a possible malfunction, it is important to be able to shut down the controller in a controlled manner to prevent damage to the using equipment. Also, conventional controller designs make it difficult to replace a SCR.
An additional concern with respect to the supply of power to using equipment is the generation of DC components within the system. DC current, for example, can damage not only transformers upstream from the controller, but also downstream inductive loads.
Because of the amount of heat generated within an enclosure, the SCR power controllers are either air cooled or water cooled. The SCR's are the primary heat generators within the enclosure. If air cooled, a fan must be mounted on the enclosure to blow air over heat sinks on which the SCR's are mounted to remove the heat generated by their operation. In standard enclosure designs, the fan is installed to one side of the enclosure so it will either pull air into or push air through the enclosure. For this purpose, a side or back panel of the enclosure is formed with a series of louvers to allow appropriate air flow. However, this type of cooling arrangement is not the most efficient Usually, unless additional fans are used to help air circulation through the enclosure, temperature increases of 25.degree. F-35.degree. F (13.degree. C. -19.degree. C.) within the enclosure are not uncommon.
Each of the foregoing indicate problems with controller designs which either add to the overall cost of the enclosure, render the controller unable to perform as efficiently as possible in supplying the network and equipment connected to it, or both. An improved design of the power controller and its installation enclosure would provide for a lower cost, more versatile installation assembly.